Nanocoral ZnO films fabricated on flexible poly(vinyl chloride) using a carrier substrate
Introduction
There is a significant urge to integrate the conventional electronic materials and devices with flexible substrates in order to enable rollable personal electronic devices, wearable electronics or the coverage of complex architectural or automotive surfaces with sensing or photovoltaic devices. As far materials for photovoltaic and sensing applications are concerned, zinc oxide has been receiving considerable attention due to its wide-band-gap-induced transparency in the visible wavelength range, high exciton binding energy of 60 meV (promoting efficient light emission at room temperatures and above) but most importantly due to the plethora of nanostructured ZnO morphologies reported in the literature. Nanorods, nanowires, nanofibers [1], [2], [3], [4], nanobelts, nanorings, nanosprings [5], nanoflowers [6] and even hierarchical nanoforests [7] have been presented. These nanocrystallite forms exhibit highly developed surfaces and are therefore regarded as desirable for light and particle absorption applications.
Most of the nanostructured ZnO both on rigid and flexible substrates is produced using solution based methods [8], [9], [10], [11], including hydrothermal growth [12], anodization [13] and electrospinning [14]. These techniques are inexpensive to apply and safe for flexible polymer substrates as the used process temperatures are lower than 150 °C–200 °C. However, their scaling for industrial applications is nontrivial with the issues of solution degradation and aging as well as solution composition and purity control. The application of cost-competitive vacuum-based fabrication processes enables to evade these problems while obtaining a high level of material purity and composition control. In particular, magnetron sputter deposition is such a technique widely used in large-area industrial coating applications (e.g. architectural glass coating, photovoltaic cell fabrication).
Until recently there were no reports on the fabrication of nanostructured nanocoral ZnO using magnetron sputtering. In 2012, papers on sputter deposition of porous Zn with subsequent annealing oxidation to ZnO were reported by our group [15] and that of Lambretti et al. [16] using DC and RF magnetron sputtering, respectively. We have chosen the use of DC sputter deposition as more cost-effective compared to RF sputtering due to lower cost of both materials and power supplies needed. Subsequently, we showed the potential of nanocoral ZnO in applications for resistive alcohol sensors and transmission-based gas sensors [17]. However, the direct application of this material on flexible polymer substrates was not possible as the fabrication procedure requires an oxygen annealing step at a temperature of 400 °C or higher while most polymers degrade at temperatures around 200 °C. Therefore, a variation of the method was developed, where a nanocoral ZnO film was fabricated on a rigid carrier substrate through DC magnetron sputter deposition with the post-deposition annealing step modified in a way enabling the detachment of the ZnO film from the carrier substrate. It was subsequently possible to transfer the film onto a selected polymer elastic substrate covered with a thin layer of adhesive.
Similar transfer technologies are often used in the microelectronic industry where it is desirable to combine materials and substrates that either have different characteristics or require different technological steps. An example of such transfer technology may be Soitec's Smart Cut™ [18], [19] using hydrogen implantation and annealing to break off in a controllable manner a thin layer of a crystalline material from its bulk and to attach it to a carrier substrate. This technology enabled the fabrication of GaN-based microwave HEMT structures on crystalline SiC on poly-SiC (SiCopSiC) substrates reducing the need for thick crystalline SiC wafers and thus cutting costs, while maintaining the high thermal conductivity of SiC [20].
In this report we describe the method of fabricating nanoporous ZnO films on flexible polymer substrates through subsequent steps of sputter deposition onto a carrier substrate, high temperature annealing and transfer onto the polymer.
Section snippets
Experimental details
Fig. 1 depicts the schematic of the sample fabrication process. In the first step, porous Zn films were grown on a Si (111) carrier substrate using reactive sputtering of a 3″, 4N-pure Zn target under 80 W DC power in a mixed Ar–O2 atmosphere. The gas flow rates of Ar and O2 were 10 sccm and 2 sccm, respectively and the total gas pressure was 1.5 mTorr (0.2 Pa). The deposition was carried out for 1 h. The details of the growth procedure performed in a Surrey NanoSystems Gamma 1000C reactor can be
Results and discussion
From the SEM image presented in Fig. 3a it is evident, that the surface of the nanocoral film reflected the flat interface with the Si carrier, at the same time maintaining the nanoporous structure. The porosity of the film surface was calculated using image processing of a SEM image (see Fig. 3b) using the Otsu method of binarization which chooses a global threshold that minimizes the interclass variance of the two resulting regions [21] with subsequent morphological opening using a
Conclusions
In summary, we report a vacuum-based fabrication technology of nanocoral ZnO films on polymer elastic substrates using a three step approach, consisting of (1) deposition of nanostructured Zn onto a carrier substrate, (2) high temperature oxidation of Zn to ZnO and (3) transfer to an elastic substrate. In the oxidation step the ZnO film was detached from the carrier substrate by means of strain engineering (i.e. using appropriate temperature rise rate and heater geometry). By applying
Acknowledgments
This study was partially supported by the European Union within the European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08 “Nanobiom”). One author (M.W.) also acknowledges the support of the Ventures Programme of the Foundation for Polish Science operated within the Innovative Economy, European Regional Development Fund.
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